Broad area lasers include a broad area gain region within at least a portion of the resonant cavity of the laser, instead of having only a narrow single spatial mode waveguide that extends over the entire length of the cavity, and thus is capable of achieving higher output power than conventional single mode lasers. The broad area gain region, with a width greater than about 5 .mu.m, is capable of supporting multiple spatial modes of propagation, with the light generally propagating freely within the broad area gain region, any wave-guiding of the light, such as from gain guiding effects of current injection, usually being incidental. Thus, if actual operation of a broad area laser in a single spatial mode is desired, then some other mechanism, such as a single mode waveguide spatial filter, must also be supplied to the laser structure in order to select the desired spatial mode.
In semiconductor lasers, the gain characteristics of a laser's active medium combined with the characteristics of the laser's resonant optical cavity frequently permit the laser oscillation of any one of several possible longitudinal modes, each mode corresponding to a different wavelength of the light output from the laser. Unfortunately, which particular mode or combination of modes lases at any particular moment may depend on factors such as the temperature of the laser and the amount of current injected into the laser's active medium, so that the laser can "hop" from one longitudinal mode to another during operation as those conditions change, for example, when directly modulating the laser. Semiconductor lasers that are susceptible to such longitudinal mode hopping are unsuitable for many applications requiring single frequency outputs, such as fiberoptic communications.
In addition to the many possible longitudinal modes, broad area lasers may also operate in any one of several transverse or spatial modes. Unfortunately, the spatial modes generated within the optical cavity are not necessarily mutually coherent. As the light generated by the laser active medium propagates along a path within the optical cavity and increases in light intensity, multiple independent light filaments may be created. The free carrier density at the center of each filament is decreased as a result of the increased intensity along the path of the light filament. This decrease in charge density causes an increase in the refractive index at the center of the mode resulting in the formation of a stable waveguide for the filament. Unfortunately, each filament is stable only within itself and may not be coherent with other regions of the broad area laser. As with numerous longitudinal modes, the filamentary laser light resulting from the generation of more than one spatial mode makes the resultant output beam unsuitable for many commercial uses.
Some broad area semiconductor lasers include a grating within the optical cavity. The grating is tuned such that feedback in the optical cavity occurs only for light of a particular wavelength. However, light of differing wavelengths may still resonate within the cavity due to effects such as parasitic Fabry-Perot oscillation. Additionally, numerous spatial modes may still occur in such systems.
In U.S. Pat. No. 5,103,456, Scifres et al. describe a master oscillator power amplifier (MOPA) device with a broad beam output. An angled grating in the device couples light unidirectionally from a single transverse mode laser oscillator into a broad area amplifier laterally disposed relative to the oscillator's waveguide. The laser oscillator typically has a 1-5 .mu.m wide real refractive index waveguide that supports a single spatial mode at least to moderate power levels (tens of milliwatts) and preferably up to 100 mW. The amplifier has a width of 1-10 mm and boosts the optical power over a length of about 0.5-4 mm to about 1200 mW-2500 mW. The angled grating is oriented to minimize feedback from the amplifier into the laser oscillator.
In U.S. Pat. No. 4,658,403, Takiguchi et al. describe a diode laser in which a grating is used to optically couple two offset waveguides. Light generated in one of the waveguides, and having a particular wavelength, is reflected by the grating into the other waveguide. In so doing, Takiguchi et al. attempt to suppress unwanted oscillations of light.
It is an object of this invention to provide a broad area semiconductor laser which is able to generate a coherent single wavelength and single spatial mode beam without producing mutually incoherent light filaments.
It is a further object of the present invention to provide a broad area semiconductor laser which is suitable for use in a monolithic array of semiconductor lasers.
It is still another object of the present invention to provide a broad area semiconductor laser which can be used in a master oscillator power amplifier (MOPA) to produce a coherent high power output beam.